This invention relates to television camera systems in which a video signal is developed from one or more image pickup tubes and, in particular, to suppression circuits for eliminating from the video signal developed by the image pickup tubes the effect of illumination highlights present in the scene being viewed.
Modern day television cameras include one or more image pickup tubes for developing a video signal from the scene being viewed. The image pickup tube generally comprises an electron gun assembly for generating an electron beam which is used to scan a photoconductive target on which the light from the scene is imaged. Beam deflection coils placed around the pickup tube and driven by horizontal and vertical rate signal waveforms cause the electron beam to scan a raster on the photoconductive target to develop a video signal representative of the scene as the electron beam recharges the photoconductive target to cathode potential.
Image pickup tubes utilizing a photoconductive target on which the light from a scene is imaged, as described above, may develop levels of target voltage modulation, upon exposure to above normal illumination highlights in the scene, that cannot be recharged fully by the normal steady state beam current which is used to develop the output video signal from the image pickup tube by recharging the photoconductive target. Where the highlight exposure is relatively intense, for example, as illustrated by a point source of light in the scene, many passes or scans of the target by the beam will be required for complete recharge of the target. If the highlight in question is moving, an effect known as comet-tailing results in which the moving highlight is trailed by a tail of light. In a color camera system, the comet-tailing is particularly troublesome since the comet tails may flare up as spurious bright colors in the final display not related to the highlight or scene background. One approach to eliminating the comet-tailing effect has resulted in the development and use of an anti-comet tail (ACT) image pickup tube. In an ACT pickup tube, the electron gun utilized to form the electron beam includes a special electrode (auxiliary grid) in the form of a segmental cone placed between the gun cathode and lens electrodes. During the horizontal flyback period, the gun electrodes are pulsed under control of pulse generator circuits so as to change the focussing of the beam through the lens electrodes in a manner to significantly increase the beam current during each horizontal retrace period for recharging the target electrode, thereby eliminating the excessively discharged target areas which create the comet tails.
ACT image pickup tubes of the form described above, although successfully minimizing the comet-tailing due to illumination highlights, do have a serious disadvantage in terms of a shortened operating life. This is due primarily to the effect of the increased beam current during the flyback or retrace period which effectively decreases the cathode life and adversely affects the life and performance of the photocathode target electrode. An ACT image tube may also be operated in a non-anti-comet tail mode which will extend the pickup tube life. The present invention utilizes the advantages of both modes of operation of a ACT image pickup tube, thus avoiding the disadvantages by providing an image suppression circuit which automatically provides normal ACT image pickup tube pulsing "on demand", that is, only in the presence of illumination highlights, thereby significantly extending the operating life of the ACT pickup tube, while advantageously utilizing the anti-comet tail features of the pickup tube.